The scale on a map is 10cm : 1km. If the distance between two cities is 3km, how far apart in cm are the two cities on the map?
Solution: The scale means that every 10cm on the map corresponds to an actual distance of 1km. An actual distance of 3km is the same as 3 $\cdot$ 1km. The distance between the two cities on the map is 3 $\cdot$ 10cm, or 30cm.